deer: implement Deserialize for core::marker (#2379)

* chore: convert from #1875

* test: `PhantomData`

libs/deer/src/impls/core.rs

@@ -4,6 +4,7 @@ mod bool;
 mod cmp;
 mod floating;
 mod integral;
+mod marker;
 mod mem;
 mod non_zero;
 mod option;

libs/deer/src/impls/core/marker.rs

@@ -0,0 +1,45 @@
+use core::marker::PhantomData;
+
+use error_stack::{Result, ResultExt};
+
+use crate::{
+    error::{DeserializeError, VisitorError},
+    Deserialize, Deserializer, Document, Reflection, Schema, Visitor,
+};
+
+struct PhantomDataVisitor<T: ?Sized>(PhantomData<T>);
+
+impl<'de, T: ?Sized> Visitor<'de> for PhantomDataVisitor<T> {
+    type Value = PhantomData<T>;
+
+    fn expecting(&self) -> Document {
+        Self::Value::reflection()
+    }
+
+    fn visit_none(self) -> Result<Self::Value, VisitorError> {
+        Ok(PhantomData)
+    }
+
+    fn visit_null(self) -> Result<Self::Value, VisitorError> {
+        Ok(PhantomData)
+    }
+}
+
+pub struct PhantomDataReflection;
+
+impl Reflection for PhantomDataReflection {
+    fn schema(_: &mut Document) -> Schema {
+        // TODO: this is also optional (none)
+        //  currently we're unable to express that constraint (something for 0.2)
+        Schema::new("null")
+    }
+}
+
+impl<'de, T: ?Sized> Deserialize<'de> for PhantomData<T> {
+    type Reflection = PhantomDataReflection;
+
+    fn deserialize<D: Deserializer<'de>>(de: D) -> Result<Self, DeserializeError> {
+        de.deserialize_null(PhantomDataVisitor(Self))
+            .change_context(DeserializeError)
+    }
+}

libs/deer/tests/test_impls_core_marker.rs

@@ -0,0 +1,8 @@
+use core::marker::PhantomData;
+
+use deer_desert::{assert_tokens, Token};
+
+#[test]
+fn phantom_data_ok() {
+    assert_tokens::<PhantomData<u64>>(&PhantomData, &[Token::Null]);
+}